1. Field of the Invention
This invention relates to a light detecting device for use in light output monitoring devices of semiconductor lasers, receiving devices of optical communication systems,
2. Prior Art
FIG. 1A is a top view of a structure of a conventional light detecting device, and FIG. 1B is a sectional view along the line I--I in FIG. 1A. As shown, the conventional light detecting device comprises a first conduction-type semiconductor substrate 1 with a first electrode 8 formed on the underside; a first conduction-type semiconductor crystal layer 2 including a light absorbing layer; and a second conduction-type first region 3 formed in the first conduction-type semiconductor crystal layer 2 by selectively diffusing a dopant. Thus formed is a pin photodiode structure. pin photodiode structure includes an n-layer (or a p-layer) provided by the semiconductor substrate 1, a p-layer (or an n-layer) provided by a first region 3, and light detecting region 10 provided by the pn junction (the depletion layer or the i layer). A second electrode 5 is provided on the first region 3 in the semiconductor crystal layer 2. The top of the first region 3 inside the electrode 5 is covered with a reflection preventive film 6, and the top of semiconductor crystal layer 2 outside the electrode 5 is covered with a device protective film 9.
In the semiconductor device of the above-described structure, when an reverse bias is applied, an electric field is generated in the depletion layer. Electrons and holes generated by incident light on a light detecting region 10 are divided respectively to the first conduction-type semiconductor substrate 1 and to the second conduction-type region 3 and are accelerated. Thus a photocurrent can be taken outside.sub.7 and an optical signal can be detected.
In the above-described structure of FIG. 1A and 1B, when light is incident on the light detecting region 10, photo-carriers are generated in the depletion layer, and a good response characteristic can be obtained. But when light is incident outside the light detecting region 10, due to density gradient the generated carriers are diffused to reach the depletion layer, and are taken out in a photocurrent. The transfer of the diffused carriers is slow. When the carriers reach the light detecting region 10, adversely a tail is generated at the last transition of a light-pulse-responding waveform.
In the application of the above-described light detecting device to the optical communication, light exiting an optical fiber is focused so as to enter the light detecting region 10. In a case that a part of the light leaks outside the light detecting region 10, it leads to a lowered response speed of the light detecting device for the above-described reason. Especially in a light detecting device of high response speed, since an area of the light detecting region 10 is made small to decrease a junction capacitance, light is incident by a higher ratio on the outside of the light detecting region 10, adversely a diffused carrier component resulting in a lowered response speed is increased, which leads to a decrease in a response speed of the device.
In the case that, to keep a light output of a semiconductor laser constant, a drive current of the semiconductor laser is feedback-controlled on the basis of light emitted from the rear end surface of the semiconductor laser, if light emitted by the semiconductor laser adversely spreads even outside the light detecting region 10 of the light detecting device, the diffused carriers, as described above, a carrier component which delays a response speed, which affects the feedback control.